Breast cancer is second only to lung cancer as a cause of cancer mortality among women, and more than 20% of hormone receptor positive breast cancers in women with local or regional disease are expected to recur. Little is known, however, about why some cancers recur while others do not, and breast cancer survivors often lack information on lifestyle or other changes they could make to reduce their likelihood of recurrence. Given the prevalence of breast cancer, the identification of safe, effective agents to reduce recurrence is of major importance and would have high impact on cancer treatment. Statins are among the most commonly prescribed drug classes in the U.S. and are the focus of growing interest in cancer research. Laboratory studies have shown that statins have various antineoplastic properties, suggesting possible associations between statin use and cancer incidence and outcomes. Observational studies of the association of statins with incident breast cancer have produced inconsistent results. Only a few studies to date, however, have attempted to determine if a statin/breast cancer association exists for treatment outcomes. Their results look promising, but some of these studies were hampered by significant limitations (i.e., limited statistical power and failure to evaluate important potential confounders). To address this important gap in knowledge, we propose to expand data collection and conduct additional analyses using the subjects included in a currently-funded R01 case-control study, CYP2D6 gene variants and effectiveness of adjuvant tamoxifen in breast cancer. That study, with a population drawn from the large health plan member population of the Northwest region of Kaiser Permanente, is investigating the association between the major variants of the CYP2D6 gene and breast cancer recurrence in women receiving tamoxifen for hormone receptor-positive breast cancer. In that study, we are reviewing medical records and pathology materials to gather demographic, behavioral, and pathologic information on risk factors for breast cancer recurrence. Because the parent study is designed to examine multiple risk factors for recurrent breast cancer, we can cost-effectively expand data collection and analysis efforts to investigate the role of statins in breast cancer recurrence, after adjustment for appropriate confounding factors. If this and other studies find that statins are associated with a reduction in breast cancr recurrence and death, research results can be easily translated into clinical practice.